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        *{padding:0; margin:0;}
        ul li{
            float:left;
            list-style: none;
            margin: 20px; 
        }
        ul li a{
            width:92px; height:92px; 
            display: block;
            text-align: center; 
            line-height: 92px;
        }
        ul li a:hover{
            background: url(images/bg.png) no-repeat;
        }
        ul li img{
            vertical-align: middle;
        }
      
        /* 
            分析
                1，插入图片要在92*92的范围内水平垂直居中
                2, 背景图显示隐藏，背景图上可以放文字，再次插入图片，放表单
        */
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